This invention generally relates to electrochemical devices. More particularly, it relates to solid oxide fuel cells.
Many different types of fuel cells are known in the art, and described in many references. In general, a fuel cell is a device in which a fuel such as hydrogen or a hydrocarbon is electrochemically reacted with an oxidant such as air or oxygen, to produce a DC electrical output. The fuel cell includes a porous anode, or fuel electrode, which enhances the rate at which electrochemical reactions occur on the fuel side. There is also a porous cathode, or oxidant electrode, which functions similarly on the oxidant side. The anode is usually based on a mixture of a metal with a ceramic, such as nickel with stabilized zirconia. The cathode is usually based on one or more ceramic materials that are doped for high electrical conductivity, such as stabilized zirconia impregnated with strontium-doped lanthanum manganate (“lanthanum strontium manganate”). In a solid oxide fuel cell (SOFC), a solid electrolyte separates the anode from the cathode. The dense electrolyte is often formed from yttria-stabilized zirconia (YSZ).
In an SOFC, the fuel flowing through the anode reacts with oxygen ions to produce electrons and water. Other reaction products may also be present, e.g., carbon monoxide and/or carbon dioxide when the fuel includes various hydrocarbons. The water and other reaction products are removed in the fuel flow stream. The oxygen reacts with the electrons on the cathode surface to form oxygen ions that diffuse through the electrolyte to the anode. The electrons flow from the anode through an external circuit, and then to the cathode. The electrolyte is a ceramic material that is a non-conductor of electrons, ensuring that the electrons must flow through the external circuit to do useful work. However, the electrolyte permits the oxygen ions to pass through from the cathode to the anode.
SOFC devices are usually operated at temperatures between about 700°-1000° C. However, in order to increase the stability, efficiency, and service life of the devices, much effort is underway to reduce this temperature. Efforts are also being made to reduce the overall thickness of the SOFC—especially its electrolyte layer. A reduction in thickness can result in numerous advantages, such as decreased electrical resistance within the cell, and a decrease in cost.
The porosity of the anode and cathode (the active components of the cell) is very important to SOFC performance. Porosity controls the transport of gaseous fuel/oxidant to the reactive sites of the cell. Porosity also controls the length of the triple-phase boundary (TPB). The “TPB” can be defined as the interface at which the electronically/ionically conductive electrodes meet both the electrolyte and the gaseous fuel/oxidant. Charge transfer for the electrodes occurs at the TPB. Thus, a larger TPB, i.e., a greater number of triple phase boundary sites, can result in a greater power density—more electron flow out of the cell and into an external circuit.
In many conventional processes, the cathode-electrolyte-anode components of an SOFC are formed by positioning the individual layers on top of each other. For example, each individual layer can be tape-cast or screen-printed according to a desired sequence. A sacrificial, pore-forming material like graphite or an organic polymer is incorporated into the anode or cathode compositions. The powdery layers (in the “green” state) are then usually sintered in some sort of furnace. Sintering causes the powdery material (e.g., YSZ) to agglomerate. Moreover, at sintering temperatures, the sacrificial material is completely burnt out, resulting in a porous structure which is quite acceptable for some end uses.
While the conventional sintering processes mentioned above are suitable for preparing some types of SOFC devices, they also have some drawbacks. For example, larger-area fuel cells, e.g., those greater than about 10 inches×10 inches (25 cm×25 cm), may undergo significant warpage when sintered.
Furthermore, sintering of the active layers tends to form relatively large particles (grains) of each species. For example, sintering of materials like nickel and YSZ in the anode can result in particle sizes of greater than about 3 microns. There is strong evidence to suggest that this relatively large particle size is not ideal for many of the future-generation SOFC devices. Within a porous anode, the pattern of large particles does not result in the number of triple-phase boundary sites which are required for greater fuel cell efficiency and power, as discussed previously.
More recently, other processes have been employed to form anodes and other layers in SOFC's. For example, promising deposition techniques which do not appear to rely on sintering are described by M. Lang et al in “Development and Characterization of Vacuum Plasma Sprayed Thin Film Solid Oxide Fuel Cells” (Journal of Thermal Spray Technology, Vol. 10(4), 2001). Lang describes the use of vacuum plasma-sprayed layers to form the device. The use of these processes is said to result in thin devices with low internal resistance. The article describes cell-operating temperatures of 750° C.-800° C., with a power density of 300-400 mW/cm2. Moreover, these types of deposition processes may minimize or eliminate some of the problems associated with sintering larger-size SOFC layers, such as warpage and cell-cracking.
While the concepts described by M. Lang et al may overcome some drawbacks of the prior art, the fuel cells described therein still appear to be deficient in some aspects. For example, the anode for the type of SOFC described by Lang appears to be too thick to be successfully incorporated into more advanced SOFC's. Moreover, the nickel-zirconia material described by Lang appears to be composed of particles which neither provide the optimum porosity, nor the optimum microstructure, for the anode layer. Thus, the SOFC's will probably fail to provide the power density required for next-generation devices.
With these challenges in mind, new developments for SOFC devices would be welcome in the art. SOFC's with improved active layer characteristics would be especially desirous, since they could lead to higher power density for the cells. Moreover, any reduction in the thickness of the SOFC would also be very useful. Furthermore, it would be very desirable if one or more of these improvements could be accomplished while still allowing the device to be made economically, and with acceptable high-temperature durability.